This invention relates to mirror constructions of the type where a specularly reflective metal layer is applied to a smooth substrate and protectively covered with a polymeric layer or coating. The invention is particularly concerned with mirrors for use in solar collectors or reflectors incorporated in lamp fixtures.
U.S. Pat. No. 4,307,150 describes and claims a solar reflector in which an opaque, specular metallized polymeric foil is protected from corrosion and weathering by a layer of an interpolymer of certain acrylate and/or methacrylate copolymers. This patent broadly hints at the use of vapor-deposited gold, silver, and copper metals to attain specularity but is primarily directed to the use of aluminum for that purpose. The aluminum is vapor-deposited on the smooth surface of a co-extruded biaxially oriented polyester foil that consists of a first polyethylene terephthalate lamina containing conventional slip agents and hence having a mildly irregular surface to facilitate winding and a second polyethylene terephthalate lamina containing no slip agent and hence having an exposed surface which is essentially optically smooth.
Silver is inherently more reflective than aluminum, the pure metals respectively reflecting approximately 98% and 91% of the light striking their surface. When incorporated in structures of the type described in U.S. Pat. No. 4,307,150, the effective reflectiveness is somewhat reduced by the presence of the protective coating, viz., to approximately 95% for silver and 85% for aluminum. The 10% greater reflectivity of silver in solar reflectors is significant, since the major cost of such reflectors resides in the hardware itself, rather than in the reflective foil which might be incorporated in them. When metallized foils of this type are utilized as reflectors in lighting fixtures, however, the 95/85 silver/aluminum relative reflectivity is repeatedly multiplied as light from the lamp in the fixture is reflected and re-reflected; as a result, perhaps a 35-50% gain in effectiveness is achieved by utilizing a reflector incorporating silver as opposed to one incorporating aluminum. (It should be noted, of course, that the white lacquered surface ordinarily found in reflectors imparts a diffuse reflectance that is less effective than any specular metal surface of equal total reflectance.)
From the foregoing discussion, it might be assumed that it would be only logical to incorporate silver in reflectors of the type described in U.S. Pat. No. 4,307,150. When this is done, however, two undesirable phenomena occur. First of all, silver is susceptible to corrosion (including the well-known tarnishing), either where pin holes in the acrylate coating occur or along the peripheral portions of the silver-coated film. Second, a thin layer of silver (unlike a thin layer of aluminum) is characterized by the presence of a spectral "window" through which ultraviolet (UV) light in the 300 to 400 nanometer wavelength region readily passes, transmission of such light peaking at approximately 325 nanometers. Ultraviolet light in this frequency range is present not only in sunlight but also in the light emitted by conventional fluorescent lamps.
Although it might be supposed that the transmission of ultraviolet light through a silver layer in constructions of the type just described would cause no problem, it has been found that such light tends to degrade the polyester substrate on which the silver is coated, releasing small bubbles of carbon dioxide gas. Since such gas cannot escape outward through the silver and acrylate protective coating, it moves inward, causing bubbles in the adhesive which commonly attaches the reflective film to a rigid support and imparting a "chicken skin" appearance that reduces the aesthetic properties of the material.
Attempts have been made to solve the problems just described by incorporating corrosion inhibitors and/or UV absorbers in the protective acrylate coating layer. Although the corrosion inhibitors ameliorate the corrosion problem, they frequently impart an unacceptable color and, in any event, offer little or no help in blocking the ultraviolet light. On the other hand, when ultraviolet light absorbers are included in the protective acrylate coating, degradation of the polyester support is prevented but the corrosion of the silver is actually exacerbated.